1. Field of the Invention
The present invention relates generally to defibrillation processes and waveforms, and more particularly, to truncated and optimally short capacitance-discharge pulses that are attuned to the natural time constants of the system and of the heart, and are delivered by an implantable cardioverter-defibrillator (ICD) or implantable defibrillator.
2. Description of the Prior Art
Present-day defibrillators and ICDs deliver defibrillation shocks of high energy with pulse durations ranging from 6 to 12 milliseconds. These pulses may be monophasic (consisting of a single pulse of a single polarity), as in FIGS. 1A and 1B. Also, the pulses may be biphasic (consisting of a pair of contiguous opposite-polarity pulses), as in FIG. 2, in which case the duration of the first pulse is from 6 to 12 milliseconds. A sampling of product available currently from five vendors is given in Table 1, with an indication of the properties of the pulses they deliver:
TABLE 1 ______________________________________ Duration Duration Vendor Waveform phase 1 phase 2 ______________________________________ Ventritex Monophasic 6 ms Ventritex Biphasic 6 6 ms Telectronics Monophasic * Medtronic Monophasic Programmable** Intermedics Biphasic 6.5 3.5 CPI Monophasic 65% tilt*** ______________________________________ *The Telectronics output is 4, 6, 8, or 12 ms depending on the energy setting for their model 4202 and 4203. The model 4210 has widths of 4, 6, and 11.5. **Pulse duration can be selected by the implanting physician, but the recommendation is to adjust duration so that a "tilt" of 0.65 obtains, where tilt = (V.sub.initial - V.sub.final)/V.sub.initial. For the typical value of 50 ohms for the interelectrode resistance, a tilt of 0.65 (or 65%, as it is often expressed) corresponds to 6 milliseconds. ***The pulse duration that accompanies a given tilt specification (see FIG. 1B), is a function of the interelectrode resistance and the value of the capacitor employed for the discharge. For the typical value of 50 ohm for the interelectrode resistance, and the value of 140 microfarads employed by CPI, the resulting pulse duration is 7 milliseconds.
While it is technically possible to program some present-day defibrillation systems for delivery of pulses with a duration shorter than 6 milliseconds, the manufacturers universally recommend using pulses above that range. The novelty, nonobviousness, and benefits of designing systems that deliver only shocks with durations in the range below 6 milliseconds are described in the Summary of the Invention below.
Determining Cardiac Chronaxie. A study and analysis of prior art data on tissue stimulation and defibrillation indicates that the conventionally recommended range of pulse durations exceeding 6 milliseconds is wide of the mark, as is shown here and also in the Summary of the Invention. The subject has a long history. In the late 19th century, Weiss found a linear relationship between the amount of charge needed for the stimulation of tissue by means of an electrical pulse, and the duration of the pulse. [G. Weiss, "Sur la Possibilite de Rendre Comparable entre Eux les Appareils Suivant a l'Excitation Electrique", Arch. Ital. de Biol., Vol. 35, p. 413, 1901.] His pulse generator, the ballistic-rheotome, comprised a dc source and a rifle shot of known velocity that first cut a shunting wire and then a series wire with a known distance between the wires, thus initiating and then ending a rectangular pulse of current. [H. Fredericq, "Chronazie: Testing Excitability by Means of a Time Factor", Physiol. Rev., Vol. 8, p. 501, 1928.] He reported that the charge Q required for stimulation by a pulse of duration d was given by: EQU Q=k.sub.1d +k.sub.2 Eq. 1
Subsequently, Lapicque divided the Weiss equation by d, thus obtaining the average current required for stimulation [L. Lapicque, "Definition Experimentelle de l'excitabilite," Proc. Soc. de Biol., Vol. 77, p. 280, 1909.], which can be written: EQU I.sub.ave =K.sub.1 +(K.sub.2 /d) Eq. 2
Lapicque also defined two useful terms. The current I.sub.r that would suffice for tissue stimulation by a pulse of infinite duration, he termed the rheobase. Shortening the pulse required progressively more current, and the pulse duration that required a doubling of current for excitation, or 2I.sub.r, he termed the chronaxie, d.sub.c. Placing 2I.sub.r and d.sub.c into Eq. 2 in place of I.sub.ave and d yields EQU d.sub.c =K.sub.2 /K.sub.1 Eq. 3
Factoring I.sub.r out of Eq. 2, and then making use of Eq. 3 yields EQU I.sub.ave =I.sub.r (1+d.sub.c /d). Eq. 4
Lapicque's model described cell stimulation, rather than defibrillation. But in 1978, Bourland, et al., demonstrated that defibrillation thresholds in dogs and ponies followed the Weiss-Lapicque model, provided average current is used in the exercise. [J. D. Bourland, W. Tacker, and L. A. Geddes, "Strength-Duration Curves for Trapezoidal Waveforms of Various Tilts for Transchest Defibrillation in Animals," Med. Instr., Vol. 12, p. 38, 1978.] In another paper, the same workers (with others) showed that average current, I.sub.ave, is a useful measure of defibrillation effectiveness for time-truncated pulses of a given duration (see FIG. 1A) through a substantial range of durations, from 2 to 20 milliseconds. [J. D. Bourland, W. Tacker, and L. A. Geddes, et al., "Comparative Efficacy of Damped Sine Waves and Square Wave Current for Transchest Defibrillation in Animals," Med Instr., Vol. 12, p. 42, 1978.] In other words, so long as the "tail" of a capacitor-discharge pulse is eliminated, its effectiveness is only a little dependent upon waveform details.
U.S. Pat. No. 4,708,145 to Tacker, Jr., et al. illustrates a representative patent for controlling cardiac ventricular fibrillation.